Acid catalysts prepared by impregnation of tungstophosphoric acid solutions on different supports

Tungstophosphoric acid (TPA) in 50% v/v ethanol-water solutions was used to impregnate carbon, TiO2, SiO2 and gamma-Al2O3 (Spheralite and Akzo) at 20 degrees C. The solutions were characterized by UV-visible and nuclear magnetic resonance spectroscopies and the results indicated that the main species present was the undegraded anion of the acid both before and after the contact with carbon, TiO2, SiO2 and gamma-Al2O3 Akzo. However, the nuclear magnetic resonance spectrum of the solution in contact with gamma-Al2O3 Spheralite allowed to observe a tungstodiphosphate anion. X-ray diffraction of the impregnated solids dried at 70 degrees C showed the same diffraction patterns as the supports, possibly owing to a high dispersion of non-crystalline species. The Fourier transform infrared and nuclear magnetic resonance spectroscopies of carbon, TiO2 and SiO2 impregnated with TPA showed that the species present was the undegraded [PW12O40](3-) anion. When using carbon and TiO2, the supported acid was thermally stable up to 425 degrees C, similarly to the bulk acid, whereas on SiO2 support, the anion underwent partial degradation from 365 degrees C onwards. In TPA-impregnated gamma-Al2O3 samples dried at 70 degrees C, diffuse reflectance and nuclear magnetic resonance spectroscopies allowed us to observe a partial degradation of the anion, the extent of which was not appreciably increased by the subsequent calcination. The specific activity of the catalysts in the isopropanol dehydration reaction showed that TPA supported on carbon, TiO2 and SiO2 present higher acidity than the bulk acid, unlike in alumina where the partially degraded species gave a lower acidity. (C) 1998 Elsevier Science B.V.